In this paper, we introduce an implementation for detailed monitoring of user actions on web pages. It addresses the problem that the log data recorded by standard web servers is not sufficient for the tracking of users on AJAX websites, e.g. to conduct a usability test. Using standard web technologies, our HTTP proxy can record very detailed usage information, such as mouse movements, clicks, key presses and scrolling, together with the exact HTML DOM tree objects involved. As we show in several case studies, the tracking also works across multiple websites, none of which needs to be under our control. This approach is much less invasive than previous efforts: The test person does not need to install software on her computer, and in certain operation modes, no configuration changes at all are required on her computer. Our research indicates that if the technology described in this paper is employed, arbitrary visitors of a website are more likely to take part in a usability test offered by that site - this facilitates recruiting test participants over the Internet. More information on UsaProxy